I believe you are referring to a vertical forgrip which is not allowed in a pistol. It would possibly reclassify it as an AOW. Technically the tac14 is not a pistol by ATF rules though. It is transferred under the same regulated firearm rules in MD which is confusing.

To my knowledge you can add a pistol grip if you like and can also add a brace legally. There are companies that sell assemblies with both. Adding the grip and brace changes the overall length which may reclassify the firearm as a pistol under ATF rules.

Clear a mud right? IANAL.

I meant like a hogue pistol grip. Does transfer as a pistol needing an HQL or does it transfer just as a regulated firearm like an ar15 receiver?